Avoid busy polling in the reader thread

[RinHizakura]

We'll observe 100% CPU usage by the reader thread when the GDBSTUB is running, and the reason is because the socket_reader keeps polling the status variable.

In #10, @fabiomurer tried to introduce a condition variable approach to solve; however, due to the wrong synchronization, the gdbserver can sometimes fail to interrupt the gdbstub. Specifically, we need to verify these behaviors for the proposed fix:

• We need to make sure the emulator can be paused if sending an interrupt(like CTRL+C)
• We must ensure the reader thread can end if the emulator stops.

[RinHizakura]

Instead of using the condition variable. I'm considering whether epoll can be a better way to fix. We'll need to explore more.